I. What is a PLC?
A PLC is an intelligent controller, a computer (PC). PLC stands for Programmable Controller.
The definition is: a digital electronic system designed specifically for industrial applications. It employs a programmable memory to store programs, execute user-oriented instructions such as logic operations, sequential control, timing, counting, and arithmetic operations, and control various types of machinery or production processes through digital or analog input/output. PLCs and their related peripherals should be designed for easy integration into industrial control systems and easy expansion of functionality.
II. Features (Advantages) of PLC
1. High reliability and strong anti-interference ability
2. Fully functional, well-equipped, and highly adaptable.
3. Easy to learn and simple to operate
4. The system is easy to design, maintain, modify, and build.
5. Small size, light weight, and low energy consumption
III. Application Areas
PLCs are widely used in industries such as steel, petroleum, chemical, power, building materials, machinery manufacturing, automobile, transportation, and cultural entertainment due to their electromechanical integrated control, storage logic replacing external wiring, strong data processing capabilities, and enhanced communication technology. They can achieve the following control functions.
1. Logic control of on/off quantities
It replaces traditional relay circuits to achieve logic control and sequential control. It can be used for controlling single devices, as well as for multi-machine group control and automated production lines. Examples include injection molding machines, printing presses, stapling machines, combination machine tools, grinding machines, packaging production lines, and electroplating production lines.
2. Analog control
In industrial production processes, many continuously changing quantities, such as temperature, pressure, flow rate, liquid level, and speed, are analog quantities. To enable programmable logic controllers (PLCs) to process analog quantities, analog-to-digital (A/D) and digital-to-analog (D/A) conversions must be implemented. PLC manufacturers produce matching A/D and D/A conversion modules to enable PLCs for analog control.
3. Motion control
PLCs can be used to control circular or linear motion. In terms of control mechanism configuration, early PLCs directly connected position sensors and actuators using digital I/O modules. Now, dedicated motion control modules are generally used, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors. Almost all major PLC manufacturers worldwide offer products with motion control capabilities, and these are widely used in various machinery, machine tools, robots, elevators, and other applications.
4. Process control
Process control refers to the closed-loop control of analog quantities such as temperature, pressure, and flow rate. As an industrial control computer, a PLC can program various control algorithms to complete closed-loop control. PID control is a commonly used method in general closed-loop control systems. Large and medium-sized PLCs all have PID modules, and many small PLCs now also have this function. PID processing typically involves running dedicated PID subroutines. Process control has very wide applications in metallurgy, chemical engineering, heat treatment, boiler control, and other fields.
5. Data Processing
Modern PLCs possess mathematical operations (including matrix operations, function operations, and logical operations), data transmission, data conversion, sorting, table lookup, and bit manipulation functions, enabling them to acquire, analyze, and process data. This data can be compared with reference values stored in memory to perform specific control operations, or it can be transmitted to other intelligent devices via communication functions, or printed into tables. Data processing is generally used in large-scale control systems, such as unmanned flexible manufacturing systems; it can also be used in process control systems, such as some large-scale control systems in the paper, metallurgical, and food industries.
6. Communication and networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of computer control, factory automation networks are developing rapidly, and PLC manufacturers are paying close attention to the communication functions of their PLCs, launching their own network systems. Newly manufactured PLCs all have communication interfaces, making communication very convenient.
IV. PLC Structure and Functions of Each Component
Programmable logic controllers (PLCs) come in various structures, but their general principles are basically the same, all based on a microprocessor. They typically consist of a central processing unit (CPU), memory (RAM, ROM), input/output units (I/O), a power supply, and a programmer.
1. Central Processing Unit (CPU)
The CPU, as the core of the entire PLC, plays a command and control role. A CPU typically consists of control circuits, an arithmetic logic unit (ALU), and registers. These circuits are usually packaged on a single integrated circuit chip. The CPU connects to the memory unit and input/output interface circuits via the address bus, data bus, and control bus. The CPU's functions include: reading instructions from memory, executing instructions, fetching the next instruction, and handling interrupts.
2. Memory (RAM, ROM)
Memory is primarily used to store system programs, user programs, and working data. The memory storing system software is called system program memory; the memory storing application software is called user program memory; and the memory storing working data is called data memory. Commonly used memory types include RAM, EPROM, and EEPROM. RAM is a read-write random access memory that stores user programs and generates user data areas. User programs stored in RAM can be easily modified. RAM memory is a high-density, low-power, and inexpensive semiconductor memory that can be powered by lithium batteries. It effectively retains stored information when power is lost. EPROM and EEPROM are read-only memories. These types of memory are used to store system management programs and application programs.
3. Input/Output Units (I/O Units)
The I/O unit is essentially the interface component that transmits input and output signals between the PLC and the controlled object. I/O units offer excellent electrical isolation and filtering capabilities. Input devices connected to the PLC's input interface include various switches, buttons, sensors, etc. The PLC's output control devices are often solenoid valves, contactors, and relays. Relays come in AC and DC types, high-voltage and low-voltage types, and voltage and current types.
4. Power supply
The PLC power supply unit includes the system's power supply and backup battery. Its function is to convert external power into internal operating voltage. The PLC also contains a regulated power supply for powering the PLC's CPU and I/O units.
5. Programmer
A programmer is the most important peripheral device for a PLC. It is used to load user programs into the PLC's memory, check and modify programs, and monitor the PLC's operating status. Furthermore, by adding appropriate hardware interfaces and software packages to a personal computer, PLCs can be programmed using the computer itself. Using a microcomputer as a programmer, ladder diagrams can be directly created and displayed.
V. Working Principle of PLC
The PLC uses a cyclic scanning working mode. In the PLC, the user program is stored in sequence. The CPU starts executing the program from the first instruction and returns to the first instruction when it encounters the end symbol. This process is repeated continuously.
The PLC scanning process is divided into several stages: internal processing, communication operation, program input processing, program execution, and program output. The time required for one complete scan is called the scan cycle. When the PLC is in a stopped state, it only performs internal processing and communication operations. When the PLC is in a running state, it continuously cycles through internal processing, communication operation, program input, program execution, and program output.
1. Input Processing
Input processing, also known as input sampling, involves sequentially reading the on/off status of all input terminals and storing the information in the corresponding image register in memory. The input image register is then refreshed. Next, the program execution phase begins. During program execution, the input image register is isolated from the outside world; even if the input signal changes, its contents remain unchanged until the input processing phase of the next scan cycle.
2. Program Execution
According to the PLC ladder diagram program scanning principle, the program is scanned line by line in a left-to-right, top-to-bottom sequence. When a program jump instruction is encountered, the jump address is determined based on whether the jump condition is met. When the user program involves input/output states, the PLC reads the corresponding input terminal state from the input image register, reads the corresponding image register from the output image register, performs logical operations according to the user program, and stores the results in the relevant device registers. For each device, the contents stored in the device image register change as the program executes.
3. Output Processing
After the program finishes execution, the state of the output image register, i.e. the Y register in the device image register, is transferred to the output latch during the output processing stage. Through the isolation circuit, the power amplifier circuit is driven, so that the output terminal outputs control signals to the outside world to drive the external load.
